1. Field of the Invention
The present invention relates to apparatus and methods for thermally processing substrates, and in particular semiconductor substrates with integrated devices or circuits formed thereon.
2. Description of the Prior Art
The fabrication of integrated circuits (ICs) involves subjecting a semiconductor substrate to numerous processes, such as photoresist coating, photolithographic exposure, photoresist development, etching, polishing, and heating or “thermal processing.” In certain applications, thermal processing is performed to activate dopants in doped regions (e.g., source and drain regions) of the substrate. Thermal processing includes various heating (and cooling) techniques, such as rapid thermal annealing (RTA) and laser thermal processing (LTP). Where a laser is used to perform thermal processing, the technique is sometimes called “laser processing” or “laser annealing.”
Various techniques and systems for laser processing of semiconductor substrates have been known and used in the integrated circuit (IC) fabrication industry. Laser processing is preferably done in a single cycle that brings the temperature of the material being annealed up to the annealing temperature and then back down to the starting (e.g., ambient) temperature.
Substantial improvements in IC performance are possible if the thermal processing cycles required for activation, annealing, etc. can be kept to a millisecond or less. Thermal cycle times shorter than a microsecond are readily obtained using radiation from a pulsed laser uniformly spread over one or more circuits. An example system for performing laser thermal processing with a pulsed laser source is described in U.S. Pat. No. 6,366,308 B1, entitled “Laser Thermal Processing Apparatus and Method.” However the shorter the radiation pulse, the shallower the region that can be heat-treated, and the more likely that the circuit elements themselves will cause substantial temperature variations. For example, a polysilicon conductor residing on a thick, field-oxide isolator is heated much more quickly than a shallow junction at the surface of the silicon wafer.
A more uniform temperature distribution can be obtained with a longer radiation pulse since the depth of heating is greater and there is more time available during the pulse interval for lateral heat conduction to equalize temperatures across the circuit. However, it is impractical to extend laser pulse lengths over periods longer than a microsecond and over circuit areas of 5 cm2 or more because the energy per pulse becomes too high, and the laser and associated power supply needed to provide such high energy becomes too big and expensive.
An alternative approach to using pulsed radiation is to use continuous radiation. An example thermal processing apparatus that employs a continuous radiation source in the form of laser diodes is disclosed in U.S. patent application Ser. No. 09/536,869, entitled “Apparatus Having Line Source of Radiant Energy for Exposing a Substrate,” which application was filed on Mar. 27, 2000 and is assigned to the same assignee as this application. Laser diode bar arrays can be obtained with output powers in the 100 W/cm range and can be imaged to produce line images about a micron wide. They are also very efficient at converting electricity into radiation. Further, because there are many diodes in a bar each operating at a slightly different wavelength, they can be imaged to form a uniform line image.
However, using diodes as a continuous radiation source is optimally suited only for certain applications. For example, when annealing source and drain regions having a depth less than say one micron or so, it is preferred that the radiation not be absorbed in the silicon beyond this depth. Unfortunately, the absorption depth for a typical laser diode operating at wavelength of 0.8 micron is about 20 microns for room temperature silicon. Thus, in thermal processing applications that seek to treat the uppermost regions of the substrate (e.g., shallower than say one microns), most of the diode-based radiation penetrates into a silicon wafer much farther than required or desired. This increases the total power required. While a thin absorptive coating could be used to reduce this problem, it adds complexity to what is already a rather involved manufacturing process.